Many piping system applications in chemical and natural resource recovery industries involve the handling of corrosive, erosive, scaling or otherwise harsh aqueous fluids. One economic approach to handling these difficult fluids is to spin cast a fluid-resistant liner onto the interior of a low cost, non-fluid-resistant pipe. The pipe material, such as low carbon steel, provides structural support for the costlier and/or structurally inadequate liner. One type of fluid-resistant liner is composed of an inorganic cementitious material, such as concretes containing Portland cement.
Common concrete lining materials are composed of a variety of inorganic non-metallic fillers and cements, forming a hydraulic slurry when mixed with water. The hydraulic slurry, which can temporarily flow like a liquid or plastic, is applied to the interior surfaces of the pipe and allowed to cure (slowly hydrate or precipitate) into a rigid pipe liner. Some water based hydratable cements (e.g., Portland cement) and concrete liners made from same are subject to chemical (e.g., corrosion) and mechanical (e.g., erosive) attack by certain harsh aqueous fluids, such as geothermal brines.
The primary objectives when creating new material components which can be used to fabricate a protective pipe liner are that the components: 1) produce a slurry which can be applied to the pipe interior; 2) harden into a liner which is attached to and moves with the pipe; and 3) resist long term fluid chemical and mechanical attack. The lined pipe should also be rugged, safe, reliable, environmentally acceptable, and low in cost.
Current cements and/or concretes used to line pipe may perform some of these objectives well in some applications, but may not be suitable for other applications. For example, a current American Petroleum Institute practice (API Recommended Practice 10E) recommends a high sulfate resistant hydraulic (water-based) cement for corrosive water applications. However, problems with this type of lining material have been observed when handling corrosive geothermal brines.
Many concrete additives or admixtures are known to improve the strength and chemical stability of a water-based cement/concrete lining material. Additives providing such properties include polymers such as polystyrene. However, the water base cement is still the primary bonding agent of these additive mixtures.
A modification of the hydraulic cement/concrete lining process is to pre-coat the carbon steel before lining. An example of this technique is found in U.S. Pat. No. 4,787,936. High strength and adhesive attachment of the pre-coat is not required, since the pre-coat is encapsulated (e.g., protected from erosion) by the overlaying cementitious materials. However, the lining must still structurally withstand the environment and a separate pre-coating process step is required.
A further modification is to post-coat and/or impregnate the pre-formed cementitious liner. An example of this approach is found in U.S. Pat. No. 3,861,944. The post-coating need not bond to the steel pipe. However, the post coating and/or liner impregnation requires a separate processing step.
The wide range of in-situ properties of geothermal fluids have made them difficult to handle using these prior methods. The wide range of fluid properties is further widened during fluid processing making them sometimes more difficult to handle. Temperatures from ambient to in excess of 300.degree. C., pH's ranging from highly acidic to basic, and dissolved (and precipitated) solid contents ranging to in excess of 20% by weight of the aqueous mixture are known to cause fluid handling problems. Even if the recovery of geothermal fluids is not an objective, these difficult-to-handle fluids may have to be handled during the recovery of oil, gas, and minerals or other natural resource recovery operations.
More recently, a waterless cement (i.e., containing insufficient water to hydrate the cement), filler and polymerizable liquid mixture (termed polymer concrete) has been developed for geothermal and other difficult applications. The polymer concrete is typically composed of a solid or aggregate mixture component, such as silica sand filler and Portland cement, and a polymerizable liquid mixture component. The liquid mixture typically contains one or more monomers and polymerization additives (e.g., catalysts). The liquid mixture may include cross-linking agents, coupling agents, initiators, solvents/heat dissipators, surfactants, accelerators, and viscosity control compounds.
Because of its cost and desirable properties, some polymer concrete compositions have included styrene as a component. Polystyrene is relatively water resistant, tends to maintain its shape, and is chemically resistant to many harsh aqueous fluids, such as inorganic liquid acids or bases. However, polystyrene may lack at elevated temperature sufficient chemical resistance, strength, and/or toughness, unless co-polymerized and/or cross-linked with other reactive unsaturates. The styrene molecule has only one reactive hydrocarbon (vinyl) site, thus making the polystyrene chain once formed (i.e., the one site reacted) difficult to cross-link and/or bond strongly to aggregate particles.
In past polymer concrete compositions (as shown in U.S. Pat. No. 4,500,674), styrene is combined with at least two different co-monomers to achieve the desired chemical resistance and strength characteristics, one of which is either acrylamide or acrylonitrile. However, these reactive materials may be toxic and/or carcinogenic. They may also compromise low cost fabrication methods (e.g., high temperature mixing and/or curing may be required), broad chemical resistance, and temperature stability of the resulting liner.
In a modified approach (as shown in U.S. Pat. No. 4,231,917), another monomer forms the major polymerizable constituent instead of styrene, but styrene may be a minor constituent. The small amount of styrene is only one of many possible minority co-monomers. The minority styrene composition may further compromise the broad chemical resistance and temperature performance of a liner when compared to the styrene majority composition.
A persistent problem with these current polymer concrete compositions is the necessity of trading-off broad spectrum chemical resistance to obtain strength. In addition, none having a majority monomer of styrene avoids requiring two reactive unsaturates/co-monomers, one specified as either acrylamide or acrylonitrile. Either material adds cost, complexity and health/safety risks to the manufacturing process of a finished product.
Other problems with current polymer concrete compositions are a propensity to crack, the carcinogenic nature of acrylonitrile and acrylamide, and difficulties in solubilizing in styrene and polymerizing acrylamide. Geothermal applications can impose severe conditions such as thermal expansion, thermal shock, vibration, and two phase flow conditions. These conditions tend to crack brittle polymer concrete liners. Acrylamide is a solid at ambient temperatures, which requires high temperature to mix and co-polymerize with styrene, which is a liquid at ambient conditions. Controlling high temperature during spin casting may be particularly difficult to achieve.